Expression of heterologous DNA sequences in a plant host is dependent upon the presence of operably linked regulatory elements that are functional within the plant host. Choice of the regulatory element will determine when and where within the organism the heterologous DNA sequence is expressed. Where continuous expression is desired throughout the cells of a plant, and/or throughout development, constitutive promoters are utilized. In contrast, where gene expression in response to a stimulus is desired, inducible promoters are the regulatory element of choice. Where expression in specific tissues or organs are desired, tissue-specific promoters may be used. That is, they may drive expression in specific tissues or organs. Such tissue-specific promoters may be temporally constitutive or inducible. In either case, additional regulatory sequences upstream and/or downstream from a core promoter sequence may be included in expression constructs of transformation vectors to bring about varying levels of expression of heterologous nucleotide sequences in a transgenic plant.
As this field develops and more genes become accessible, a greater need exists for transformed plants with multiple genes. These multiple exogenous genes typically need to be controlled by separate regulatory sequences however. Further, some genes should be regulated constitutively whereas other genes should be expressed at certain developmental stages or locations in the transgenic organism. Accordingly, a variety of regulatory sequences having diverse effects is needed.
Diverse regulatory sequences are also needed as undesirable biochemical interactions can result from using the same regulatory sequence to control more than one gene. For example, transformation with multiple copies of a regulatory element may cause: homologous recombination between two or more expression systems; formation of hairpin loops caused from two copies of the same promoter or enhancer in opposite orientation in close proximity; competition between identical expression systems for binding to common promoter-specific regulatory factors; or inappropriate expression levels of an exogenous gene due to trans effects of a second promoter or enhancer.
Isolation and characterization of seed-preferred promoters and terminators that can serve as regulatory elements for expression of isolated nucleotide sequences of interest in a seed-preferred manner are needed for improving seed traits in plants.
In view of these considerations, a goal in this field has been the discovery and construction of new regulatory sequences for transgenic control of DNA constructs.